1. Field of the Invention
The present invention relates to an apparatus for polishing an end surface of a workpiece, and more particularly, to a holder for supporting an end surface of an optical fiber during a polishing being performed.
2. Description of Related Art
The optical fiber communication has been an indispensable communication tool at present and it will be in the future too. The main structure of an optical fiber connecter used for the optical fiber communication, as FIG. 1 shows, is constituted by an optical fiber 11 passing through a ferrule 12 and then being adhered to the ferrule 12 with an adhesive. The ferrule 12 can be made from plastic, glass or ceramics. A projecting spherical surface 121 of the end surface of the ferrule 12 is pressed by an elastic polishing surface and formed by way of coarse grinding, fine grinding and polishing respectively. The finished projecting spherical surface 121 must be worked as a flawless curved surface. An optical axis of the projecting spherical surface 121 can either be parallel to a central line of the optical fiber 11 or incline a small angle with the central line.
A conventional method for polishing the end surface of an optical fiber is to keep the end surface stationary, and to rotate and revolve the polishing surface in the process of polishing the end surface. It can be known from a mathematical analysis disclosed in Taiwan Patent No. 485863 entitled as “Polishing apparatus for optical fiber end surface”. If the movement way of the polishing surface is rotation plus revolution, the end surface of the optical fiber is merely arranged on the circumference of the holder. If there is only revolution without rotation, the wearing degree of every spot on the end surface is the same. The end surfaces of multiple optical fibers can be arranged on the entire holder evenly. Besides, the patent mentioned above also discloses that if the polishing surface is fabricated to have a shape of strip, one side of the polishing surface is provided with coarse polishing particles and another side of the polishing surface is provided with fine polishing particles. FIG. 2 shows that several holders 2, 3 and 4 are on a strip polishing surface 5 in which the right side 6 of the polishing surface 5 is provided with the coarse polishing particles, and the left side 7 of the polishing surface is provided with the fine polishing particles. The pressure between the end surface of the respective optical fiber on the holders and the polishing surface 5 can be uniform if a proper arrangement is performed. Hence, the holders 2, 3 and 4 are slid on the polishing surface 5 from the side with the coarse polishing particles toward the side with the fine polishing particles, and, in the mean time, the end surface of the respective optical fiber can be finished with the process of coarse grinding, fine grinding, and polishing sequentially. Thus, if the problem regarding uniform pressure between the end surface of the respective optical fiber and the polishing surface is solved, theoretically, end surfaces of a great number of optical fibers can be polished simultaneously.
Basically, an uneven pressure is arisen from the following reasons:                1. The supporting point of the holder is not on a plane of application of force which is generated from a friction between the end surface of an optical fiber and the polishing surface so that a torque is yielded to slant the holder so as to produce the uneven pressure;        2. The polishing surface not parallel to the holder may yield the uneven pressure too if the holder is fixedly clipped.        
Please refer to FIGS. 3 and 4. A conventional polishing apparatus for the end surface of an optical fiber comprises a polishing surface 21, a ferrule 22, a holder 23 and a pressing rod 24. An end surface 251, 252 of the respective optical fiber 25 is attached to the ferrule 22. When there is a relative movement between the end surface of the optical fiber 25 and the polishing surface 21, the holder 23 is subject to a transverse force at an end face thereof. But, the supporting point is at one of contact points 31, 32 right at this time such that the holder 23 is subject to a counterclockwise torque. As a result, the holder 23 becomes slanting as shown in FIG. 4 to cause the end surface 251 to be lower than the end surface 252, i.e., it causes the pressure acting on the end surface 251 is greater than on the end surface 252.
For solving the aforementioned problem, U.S. Pat. No. 5,216,846 discloses a spacer for maintaining a constant distance between the workpiece and the polishing surface. Because the polishing surface is elastic, maintaining the constant distance means maintaining a uniform pressure. In such manner, part of the uneven pressure is borne by the spacer to overcome the preceding problem to some extent, but, the torque is still there because the original structure is kept without change such that the unfavorable factor resulting in the uneven pressure is still there. Moreover, U.S. Pat. No. 6,039,630 discloses that a pressure sensor is utilized to measure the pressure instantly and compensate the unevenness of pressure by means of a method with an electronically controlled spring.
Furthermore, U.S. Pat. No. 6,077,154 and U.S. Pat. No. 5,351,445 respectively teach a design for clipping and fixing an end surface of an optical fiber as shown in FIG. 5 in which a holder 41 is combined with a plurality of optical fiber fixtures 42 for clipping optical fibers 43 respectively. The holder 41 is fixed by the fixtures 42 on the upper side of the polishing surface 45. Because the holder 41 is clipped and fixed, the problem of slanting arisen from the torque basically dose not happen. But, there is still another problem which has to be solved; because the holder 41 is clipped and fixed, and the direction which the polishing surface 45 faces to is also fixed, the holder 41 and the polishing surface 45 are not able to be accurately parallel to each other. In order to overcome the deficiency, the common practice is to adjust the fixtures 42 angularly for fixing the holder 41. But, this way is forcible and not a natural contact such that the holder 41 always is a little unparallel to the polishing surface 45. Besides, the pressure between the end surface of the optical fiber 43 and the polishing surface 45 is produced by a force exerting upwards from the bottom, and it is not only too complicated but also not suitable for automation. Furthermore, if the polishing surface is strip as shown in FIG. 2 for being slid with several holders simultaneously, it is then difficult for the end surface on each holder to keep the uniform pressure.
Furthermore, Taiwan Patent No. 222915 entitled as “Polishing Holder for optical fiber end surface”, which is granted to the present inventor, discloses two following principles for obtaining uniform pressure:                1. The holder shouldn't be clipped and fixed, and must naturally contact with the polishing surface. That is, the holder oscillates up and down with the polishing surface when the polishing surface oscillates up and down, and the average pressure of the end surface on the holder keeps uniform when the holder oscillates up and down.        2. A plane formed by the end surface in the process of polishing is a plane of application of force that is a plane constituted with a force exerted to the holder by the polishing surface. It is necessary to provide one or several supporting points for the holder because, in principle, the holder does not move or rotate with the polishing surface in the process of polishing. In order to obtain the uniform pressure, these supporting points must be on the plane of application of force to keep the holder from moving or rotating with the polishing surface.        
Please refer to FIGS. 6A and 6B. A holder designed with the two principles comprises a main body 50 having a fixing groove 51 at the lower side of the center thereof for accepting a fixing rod 52. A plurality of symmetrical accepting grooves 53 are respectively disposed close to the periphery of the main body 50. The accepting grooves 53 can be curved grooves, cylindrical grooves or elongated grooves. The main body 50 can be circular-shaped or square-shaped. The main body 50 is combined with a plurality of fixtures 500. Each fixture 500 is joined to at least one ferrule 55 for holding an optical fiber 56. A polishing surface 57 is disposed beneath the ferrules 55 and attached to the upper side of the base 58. The upper end of the fixing rod 52 is joined to a framework 59. The lower end of the fixing rod 52 is provided with a contact portion 521 with an enlarged diameter. The contact portion 521 and the fixing groove 51 are corresponding to each other as a protrusion corresponds to an indentation without relative rotation between the fixing rod 52 and the fixing groove 51. The fixing rod 52 does not exert a downward force to the main body 50. The function of the fixing rod 52 is to restrain the main body 50 from moving or rotating.
The characteristics of the above-mentioned holder is in that only the contact portion 521 of the fixing rod 52 contacts with the fixing groove 51 during the process of the end surface of the respective optical fiber being worked, and the contact plane of the end surface of the respective optical fibers 56 to the polishing surface is approximately the same as a plane of the contact zone to allow the supporting points of the main body 50 to be on a plane of application of force generated from the friction between the optical fiber 56 and the polishing surface 57. Under the circumstances, the phenomenon of the torque being yielded to slant the main body 50 and generate the uneven pressure is incapable of happening.
The counterweights 54 exerting downward forces to the main body 50 allows the end surfaces of the optical fibers 56 to naturally contact with the polishing surface 57. When the polishing surface 57 oscillates up and down, the main body 50 follows the polishing surface 57 to oscillate up and down. Therefore, the contact pressure between the end surface of the respective optical fiber 56 and the polishing surface 57 can be maintained almost all the same. Hence, the end surfaces of the optical fibers in the same batch can obtain the same appearance after polishing so as to promote the polishing speed and quality.
Please refer to FIG. 7. Another preferred embodiment of above-mentioned Taiwan Patent No. 222915 discloses a plurality of symmetrical accepting grooves 61 disposed on the periphery of a main body 60 of a holder for receiving counterweights 62. Accordingly, the end surfaces of optical fibers 63 receive a force respectively to press a polishing surface 64 such that the main body 60 is capable of contacting with the polishing surface 64 naturally. But, the main body 60 is not provided with the fixing groove 51 as the main body 50 shown in FIG. 6A, and a plurality of symmetrical fixing rods 65 are disposed under the lateral sides of the main body 60 with a projecting contact portion 651 disposed at the bottom end of each fixing rod 65 and received in a fixing groove 661 of the framework 66 respectively. A liquid discharge hole 662 is disposed at the lower end of the fixing groove 661 for draining fluid dropped into the fixing groove 661. A contact surface between the contact portion 651 and the fixing groove 661 and a contact surface between the bottoms of the optical fibers 63 and the polishing surface 64 are in the same plane such that the supporting points of the main body 60 can be on a plane of application of force generated by the bottoms of the optical fibers 63 and the polishing surface. Thus, a torque is not possible to be yielded to slant the main body 60, and the phenomenon of the uneven pressure can be avoided.
The currently used conventional polishing machine for the end surface of an optical fiber normally polishes twelve end surfaces every time, and at most forty eight or sixty four end surfaces are polished at one time. If more end surfaces have to be polished at one time under a condition of the pressure being not uniformly distributed, it needs more time to completely polish the end surface with less pressure. But, it affects the polishing efficiency substantively. Therefore, a uniform distribution of pressure is very important for polishing a great deal of end surfaces of the optical fibers.